Regenerative  refrigerator

ABSTRACT

A regenerative refrigerator includes a cylinder; a displacer provided in the cylinder; a groove pattern formed on the exterior circumferential surface of the displacer or the interior circumferential surface of the cylinder to form a first gas passage connecting one end and the other end of the exterior or interior circumferential surface, and including a groove having at least part thereof extending along a direction to cross the axial directions of the displacer to cause gas flowing from the one end to the other end in the gap between the exterior and interior circumferential surfaces to actively exchange heat with the cylinder and the displacer; a second gas passage to and from an expansion space; and a regenerator material formed of bismuth granules and provided in at least part of the second gas passage, wherein the lowest attainable temperature is 5 K to 15 K in an unloaded state.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priorityof Japanese Patent Application No. 2009-063608, filed on Mar. 16, 2009,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to regenerative refrigerators.The present invention more particularly relates to a regenerativerefrigerator capable of attaining cryogenic temperatures with thereciprocating motions of a displacer filled with a regenerator materialin a cylinder.

2. Description of the Related Art

Examples of refrigerators widely used in cryogenic ranges include aregenerative refrigerator. The regenerative refrigerator includes aregenerative heat exchanger called a regenerator. The regeneratorcontains a heat exchange material called a regenerator material in thecontainer.

A material having high specific heat at a target temperature is used asthe regenerator material. The refrigerator is used in a wide temperaturerange of room temperature to approximately 4.2 K. Accordingly, it isdesirable to select a material that has as high specific heat aspossible over the entire range. The temperature dependence of specificheat varies greatly from material to material, and no single materialcan support the entire temperature range. Accordingly, an optimumcombination of materials is used in accordance with temperature.

Further, refrigerators include those with a lowest attainabletemperature of 4.2 K used for condensing liquid helium and those used at10 K in cryopumps and the like. Those of a two-stage type having tworegenerators are often used.

Usually, 10 K refrigerators use a wire mesh of copper or stainless steelfor a first-stage regenerator and lead spheres for a second-stageregenerator. Lead has been widely used because it is higher in specificheat than other materials and has a certain degree of structuralstrength at temperatures lower than or equal to 50 K and is alsoinexpensive. (See, for example, Japanese Laid-Open Patent ApplicationNo. 3-99162.)

In the member states of the European Union, however, due to its effecton the environment, the use of lead has been strictly restricted by theRestriction of Hazardous Substances Directive or RoHS, which took effecton Jul. 1, 2006. Therefore, regenerative refrigerators using lead as aregenerator material may be subject to this restriction. Accordingly,various kinds of regenerator materials have been proposed asreplacements for lead as a regenerator material used in regenerativerefrigerators. (See, for example, Japanese Laid-Open Patent ApplicationNo. 2004-225920.)

Japanese Laid-Open Patent Application No. 2004-225920 describes an alloyof indium, bismuth, and a third material as a regenerator material tosubstitute for lead. Indium has the specific heat next highest to thatof lead at temperatures lower than or equal to 50 K. The idea is to takeadvantage of this characteristic of indium.

Indium, however, is a very soft metal and cannot be used as aregenerator material as it is. Therefore, indium is made into an alloywith bismuth and another metal to have a hardness required for aregenerator material, but is still insufficient in hardness to bepractically used as a regenerator material. Further, there is also aproblem in that indium, whose price is approximately three times theprice of lead, is too expensive to be used as a regenerator material. Inresponse to this, bismuth or an alloy of bismuth and antimony has beenproposed as a regenerator material to replace lead. (See, for example,Japanese Laid-Open Patent Application No. 2006-242484.)

As a regenerator material to replace lead, bismuth, which is also usedas a material for cosmetics, is believed to be highly safe and free ofconcern for environmental pollution, and is also inexpensive. Bismuth,however, is lower in specific heat than lead. In particular, in acryogenic environment at or below 15 K, the specific heat of bismuth issignificantly reduced. Therefore, although bismuth has goodcharacteristics in terms of safety and burdens on the environment asdescribed above, it has been believed difficult to use bismuth as aregenerator material in regenerative refrigerators for achievingcryogenic temperatures.

In order to solve this problem, it has been proposed to mix bismuth withother regenerator materials. (See, for example, Japanese Laid-OpenPatent Application No. 2006-242484.)

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a regenerativerefrigerator includes a cylinder formed of a material having a lowthermal conductivity and a high airtightness, the cylinder having acylindrical interior circumferential surface; a displacer provided inthe cylinder so as to be reciprocatable in axial directions thereof withan expansion space formed between one end of the cylinder and thedisplacer, the displacer having an exterior circumferential surfacealong a cylindrical shape of the interior circumferential surface of thecylinder, the exterior circumferential surface being slightly smaller indiameter than the interior circumferential surface; a groove patternformed on one of the exterior circumferential surface of the displacerand the interior circumferential surface of the cylinder so as to form afirst gas passage connecting a first end and a second end of the one ofthe exterior circumferential surface of the displacer and the interiorcircumferential surface of the cylinder, the groove pattern including agroove having at least a part thereof extending along a direction tocross the axial directions of the displacer so as to cause a gas flowingfrom one to another of the first end and the second end of the one ofthe exterior circumferential surface of the displacer and the interiorcircumferential surface of the cylinder in a gap between the exteriorcircumferential surface of the displacer and the interiorcircumferential surface of the cylinder to actively exchange heat withthe cylinder and the displacer; a second gas passage through which thegas is supplied to and collected from the expansion space; and aregenerator material formed of bismuth granules and provided in at leasta part of the second gas passage, wherein a lowest attainabletemperature of the regenerative refrigerator is in a range of cryogenictemperatures higher than or equal to 5 K and lower than or equal to 15 Kin an unloaded state.

The object and advantages of the embodiments will be realized andattained by means of the elements and combinations particularly pointedout in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and notrestrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a regenerative refrigeratoraccording to a first embodiment of the present invention, illustrating abasic configuration of the regenerative refrigerator;

FIG. 2 is a cross-sectional view of a two-stage regenerativerefrigerator according to a second embodiment of the present invention,illustrating a schematic configuration of the two-stage regenerativerefrigerator;

FIG. 3 is a partial cross-sectional view of a second-stage displaceraccording to the second embodiment of the present invention,illustrating a configuration of the second-stage displacer;

FIG. 4 is a graph illustrating the volumetric specific heats ofmaterials forming regenerator materials;

FIGS. 5A and 5B illustrate the characteristics of the two-stageregenerative refrigerator according to the second embodiment of thepresent invention and the characteristics of a conventional two-stageregenerative refrigerator in comparison (with no load at a compressoroperating frequency of 50 Hz), where FIG. 5A illustrates first-stagetemperature characteristics and FIG. 5B illustrates second-stagetemperature characteristics;

FIGS. 6A and 6B illustrate the characteristics of the two-stageregenerative refrigerator according to the second embodiment of thepresent invention and the characteristics of the conventional two-stageregenerative refrigerator in comparison (loaded at a compressoroperating frequency of 50 Hz), where FIG. 6A illustrates first-stagetemperature characteristics and FIG. 6B illustrates second-stagetemperature characteristics;

FIGS. 7A and 7B illustrate the characteristics of the two-stageregenerative refrigerator according to the second embodiment of thepresent invention and the characteristics of the conventional two-stageregenerative refrigerator in comparison (with no load at a compressoroperating frequency of 60 Hz), where FIG. 7A illustrates first-stagetemperature characteristics and FIG. 73 illustrates second-stagetemperature characteristics;

FIGS. 8A and 8B illustrate the characteristics of the two-stageregenerative refrigerator according to the second embodiment of thepresent invention and the characteristics of the conventional two-stageregenerative refrigerator in comparison (loaded at a compressoroperating frequency of 60 Hz), where FIG. 8A illustrates first-stagetemperature characteristics and FIG. 83 illustrates second-stagetemperature characteristics;

FIG. 9 is a graph illustrating a relationship between the bismuth sizeand the refrigerating capacity of a bismuth regenerator materialaccording to the second embodiment of the present invention;

FIG. 10 is a partial cross-sectional view of the second-stage displacerof the two-stage regenerative refrigerator according to the secondembodiment of the present invention, illustrating another configurationof the second-stage displacer;

FIG. 11 is a partial cross-sectional view of a first-stage displacer ofthe two-stage regenerative refrigerator according to the secondembodiment of the present invention, illustrating a configuration of thefirst-stage displacer;

FIGS. 12A through 12H are schematic developments illustrating groovepatterns formed on displacer surfaces according to the second embodimentof the present invention; and

FIG. 13 is a cross-sectional view of the regenerative refrigeratoraccording to the first embodiment of the present invention, illustratinganother basic configuration of the regenerative refrigerator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

However, in the case of mixing bismuth with other regenerator materialsas described above, it is difficult to determine the mixture ratio ofbismuth and other regenerator materials. Further, there is also theproblem of an increase in the prices of regenerative refrigeratorsbecause other regenerator materials, which may be used as normalregenerator materials, are expensive.

According to one aspect of the present invention, a regenerativerefrigerator may be provided that is capable of attaining cryogenictemperatures lower than or equal to 15 K while using bismuth as aregenerator material.

A description is given below, with reference to the accompanyingdrawings, of embodiments of the present invention.

FIG. 1 is a diagram illustrating a basic configuration of a regenerativerefrigerator according to a first embodiment of the present invention.The regenerative refrigerator includes a cylinder 1 and a cylindricaldisplacer 2 provided in the cylinder 1. The cylinder 1 is formed of arigid material low in thermal conductivity and high in airtightness,such as stainless steel. A helical gas passage 4 is foamed on thecylindrical exterior circumferential surface of the displacer 2. Thehelical gas passage 4 includes one or more helical groove patterns 2 aconnecting the upper end surface and the lower end surface of thedisplacer 2.

The displacer 2 has a hollow structure. A gas passage 3 is formed insidethe displacer 2. A regenerator material 5 is contained in the gaspassage 3. The regenerator material 5 has a high heat capacity atoperating temperatures. Bismuth is used as the regenerator material 5.An expansion space 6 is defined between the displacer 2 and the lowerend of the cylinder 1.

A refrigerant gas supplied from above is supplied to the expansion space6 through the gas passage 3 inside the displacer 2. Further, part of therefrigerant gas diverges from the gas passage 3 to flow through a gapbetween the displacer 2 and the cylinder 1. This diverged (part of the)gas flows downward through the helical gas passage 4 provided on theexterior circumferential surface of the displacer 2 while exchangingheat with the surfaces of the displacer 2 and the cylinder 1, to besupplied to the expansion space 6.

The refrigerant gas is expanded and cooled in the expansion space 6 withthe (upward) movement of the displacer 2. When the refrigerant gas iscollected, part of the cooled refrigerant gas flows through the gaspassage 3 to cool the bismuth regenerator material 5. A remaining partof the refrigerant gas cooled in the expansion space 6 diverges to flowupward through the helical gas passage 4 while exchanging heat with thesurfaces of the displacer 2 and the cylinder 1, and thereafter mergeswith the refrigerant gas that has flowed through the gas passage 3.

As described above, the regenerative refrigerator according to thisembodiment uses the bismuth regenerator material 5. As described above,bismuth, which is also used as a material for cosmetics, is believed tobe highly safe and free of concern for environmental pollution, and isalso inexpensive. Therefore, the bismuth regenerative material 5 ispreferable in terms of safety and environmental burdens.

However, bismuth is lower in specific heat than lead, and in particular,the specific heat of bismuth is significantly reduced in a cryogenicenvironment at or below 15 K. Therefore, it has been believed difficultto use bismuth as a regenerator material in regenerative refrigeratorsthat attain cryogenic temperatures lower than or equal to 15 K.

According to one aspect of the present invention, at the same time thatbismuth is used as the regenerator material 5, the helical gas passage 4including the one or more groove patterns 2 a is provided on theexterior circumferential surface of the displacer 2 so as to allow arefrigerant gas to pass through the helical gas passage 4. As a result,compared with a configuration where a refrigerant gas flows through onlya gas passage inside a displacer, the refrigerant gas comes intosufficient contact with the surfaces of the displacer 2 and the cylinder1. This allows more heat exchange between the surfaces of the gaspassage and the refrigerant gas.

As a result, even when bismuth, which is lower in specific heat thanconventionally-used lead, is used as the regenerator material 5 atcryogenic temperatures lower than or equal to 15 K, it is possible toimprove thermal efficiency with respect to the bismuth regeneratormaterial 5 and to improve refrigeration performance.

Next, a description given of a regenerative refrigerator according to asecond embodiment of the present invention, which is based on theabove-described first embodiment. In the following description, atwo-stage Gifford-McMahon cycle refrigerator (hereinafter referred to as“two-stage GM refrigerator”) is taken as an example of the regenerativerefrigerator of this embodiment. FIG. 2 is a schematic diagramillustrating a configuration of the two-stage GM refrigerator, whichattains cryogenic temperatures of approximately 4.2 K to approximately10 K. In the following, a description is given of the case of attaininga cryogenic temperature of approximately 10 K.

Referring to FIG. 2, a helium compressor 10 compresses helium gas toapproximately 20 Kgf/cm², and supplies high-pressure helium gas. Thehigh-pressure helium gas is supplied into a first-stage cylinder 11through an intake valve V1 and a gas passage 16. A second-stage cylinder12 is joined to the first-stage cylinder 11.

A first-stage displacer 13 and a second-stage displacer 14, which arejoined to each other, are contained in the first-stage cylinder 11 andthe second-stage cylinder 12, respectively. A shaft member S extendsupward from the first-stage cylinder 11 to be joined to a crankmechanism 15 which is in turn joined to a drive motor M.

The first-stage displacer 13 and the second-stage displacer 14 arehollow and have internal spaces (cavities), in which regeneratormaterials 17 and 18, respectively, are contained. Further, thefirst-stage displacer 13 and the second-stage displacer 14 have gaspassages 23 a and 23 b and gas passages 24 a and 24 b, respectively. Thegas passages 23 a and 23 b connect the internal space and the outside ofthe first-stage displacer 13, and the gas passages 24 a and 24 b connectthe internal space and the outside of the second-stage displacer 14.Further, a first-stage expansion space 21 is defined between thefirst-stage displacer 13 and the first-stage cylinder 11, and asecond-stage expansion space 22 is defined between the second-stagedisplacer 14 and the second-stage cylinder 12.

Usually, the first-stage cylinder 11 and the second-stage cylinder 12are formed of a material having sufficient strength, low thermalconductivity, and a capability to sufficiently block or prevent leakingof helium gas, such as stainless steel (for example, SUS304 of JapaneseIndustrial Standards). Further, the first-stage displacer 13 and thesecond-stage displacer 14 are formed of a material having low specificgravity, sufficient wear resistance, relatively high strength, and lowthermal conductivity, such as fabric-containing phenolic resin(Bakelite).

The high-pressure helium gas supplied through the intake valve V1 fromthe helium compressor 10 is supplied into the first-stage cylinder 11through the gas passage 16 to be further supplied to the first-stageexpansion space 21 through the gas passage 23 a, the regeneratormaterial 17 for the first stage, and the gas passage 23 b. Theregenerator material 17 is formed of a wire mesh or the like. Thecompressed helium gas in the first-stage expansion space 21 is furthersupplied to the second-stage expansion space 22 through the gas passage24 a, the regenerator material 18 for the second stage, and the gaspassage 24 b. The regenerator material 18 is formed of bismuth, and maybe hereinafter referred to as “bismuth regenerator material 18.” In FIG.2, the gas passages 23 a and 23 b and the gas passages 24 a and 24 b arefunctionally illustrated to explain the flow of the refrigerant gas, andmay have actual structures different from the illustrated structures.

When the intake valve V1 is closed and an exhaust valve V2 is opened,the high-pressure helium gas in the second-stage cylinder 12 and thefirst-stage cylinder 11 follows the intake path in the reverse directionto be collected into the helium compressor 10 through the gas passage 16and the exhaust valve V2.

When the two-stage GM refrigerator of FIG. 2 is in operation, the drivemotor M rotates to vertically reciprocate the first-stage displacer 13and the second-stage displacer 14 as indicated by a double-headed arrowin FIG. 2. When the first-stage displacer 13 and the second-stagedisplacer 14 are driven downward, the intake valve V1 becomes open toallow high-pressure helium gas to be fed into the first-stage cylinder11 and the second-stage cylinder 12.

When the first-stage displacer 13 and the second-stage displacer 14 aredriven upward by the drive motor M, the intake valve V1 becomes closedand the exhaust valve V2 becomes open so that the helium gas iscollected into the helium compressor 10, and the pressure of thefirst-stage expansion space 21 in the first-stage cylinder 11 and thepressure of the second-stage expansion space 22 in the second-stagecylinder 12 are reduced. At this point, the helium gas expands togenerate coldness in the first-stage expansion space 21 and thesecond-stage expansion space 22. The cooled helium gas passes throughthe second-stage displacer 14 and the first-stage displacer 13 to becollected. During this process, the cooled helium gas cools theregenerator materials 18 and 17. (A detailed description is given belowof this cooling process.)

The high-pressure helium gas supplied in the next intake process iscooled by being supplied through the regenerator materials 17 and 18.The cooled helium gas is further cooled through its expansion. In asteady state, the first-stage expansion space 21 of the first-stagecylinder 11 is maintained at temperatures of approximately 40 K toapproximately 70 K, and the second-stage expansion space 22 of thesecond-stage cylinder 12 is maintained at cryogenic temperatures ofapproximately 9.5 K to approximately 15 K, for example.

A first-stage heat station 19 is provided around the bottom part of thefirst-stage cylinder 11 to be thermally coupled to the first-stagecylinder 11. A second-stage heat station 20 is provided around thebottom part of the second-stage cylinder 12 to be thermally coupled tothe second-stage cylinder 12. The first-stage heat station 19 is joinedto, for example, a cryopanel to cause gas molecules to be adsorbed tothe cryopanel. Further, the second-stage heat station 20 is joined to,for example, an adsorption tower containing an adsorbent such asactivated carbon to adsorb remaining gas molecules. A cryopump havingsuch a configuration is used to form a clean vacuum in sputteringapparatuses and the like.

FIG. 3 is a diagram illustrating a configuration of the second-stagedisplacer 14 of the two-stage GM refrigerator of FIG. 2. Thesecond-stage displacer 14 includes a cylindrical member 30 formed offabric-containing phenolic resin. The cylindrical member 30 has acylindrical shape open at its upper and lower ends. For example, if thesecond-stage cylinder 12 illustrated in FIG. 12 is 35 mm in insidediameter, the cylindrical member 30 is slightly smaller than 35 mm inoutside diameter and 30 mm in inside diameter. The second-stagedisplacer 14 is, for example, approximately 200 mm long in an axialdirection. A lid member 31 formed of a material such asfabric-containing phenolic resin is inserted into and bonded to thecylindrical member 30 at its lower end. A wire mesh 32 is placed on thelid member 31, and a felt plug 33 is placed on the wire mesh 32.

The bismuth regenerator material 18 consisting of bismuth is placed onthe felt plug 33. A felt plug 34 is placed on the bismuth regeneratormaterial 18. Thus, the bismuth regenerator material 18 fills in thespace between the felt plugs 33 and 34. A perforated metal 35 is placedon the felt plug 34. The perforated metal 35 is fixed to a stepped partprovided circumferentially on the internal surface of the cylindricalmember 30 at its upper end portion. A joining mechanism 36 for joiningthe second-stage displacer 14 to the first-stage displacer 13 isattached to the upper end of the cylindrical member 30.

Openings 37 forming a gas passage are provided in the sidewall of thecylindrical member 30 at the same lengthwise position as the wire mesh32 in a lengthwise direction of the cylindrical member 30. That is, thepositions of the openings 37 are level with the position of the wiremesh 32 in a vertical direction. A helical gas passage 38 of a singlehelical groove connecting the positions of the openings 37 and the upperend of the cylindrical member 30 is formed on the cylindrical exteriorcircumferential surface of the cylindrical member 30 above the openings37. For example, this groove may be approximately 2 mm in width andapproximately 0.6 mm in depth, and may have a pitch of approximately 4mm.

The cylindrical member 30 is slightly smaller in diameter below theopenings 37 than above the openings 37. The gap formed between thecylindrical member 30 and the second-stage cylinder 12 (FIG. 2) belowthe openings 37 forms a gas passage connecting the inside of thecylindrical member 30 and the second-stage expansion space 22illustrated in FIG. 2.

The gap (distance) between the exterior circumferential surface of thecylindrical member 30 and the cylindrical interior circumferentialsurface of the second-stage cylinder 12 (FIG. 2) is preferably greaterthan or equal to 0.01 mm for stable reciprocation of the second-stagedisplacer 14. Further, above the openings 37, the gap (distance) betweenthe exterior circumferential surface of the cylindrical member 30 andthe interior circumferential surface of the second-stage cylinder 12(FIG. 2) is preferably smaller than or equal to 0.03 mm in order toprevent leaking gas from flowing linearly in the axial directions.

The two-stage GM refrigerator configured as described above uses thebismuth regenerator material 5 as the regenerator material of thesecond-stage displacer 14 that generates cryogenic temperatures ofapproximately 5 K to approximately 10 K. As described above, bismuth isa regenerator material suitable in terms of safety and environmentalburdens, but is lower in specific heat than lead. FIG. 4 is a graphillustrating the volumetric specific heats of materials used asregenerator materials including bismuth. As illustrated in FIG. 4, thespecific heat of bismuth is lower than the specific heat of lead, and issignificantly reduced in a cryogenic environment at or below 10 K inparticular. Accordingly, it has been believed difficult to use bismuthfor regenerative refrigerators that attain cryogenic temperatures lowerthan or equal to 10 K.

The inventors of the present invention have diligently studied aregenerative refrigerator that attains cryogenic temperatures lower thanor equal to 10 K while using bismuth as a regenerator material, and havesucceeded in attaining cryogenic temperatures lower than or equal to 15K while using bismuth as a regenerator material by forming the helicalgas passage 38 on one of the cylindrical exterior circumferentialsurface of the second-stage displacer 14 and the cylindrical interiorcircumferential surface of the second-stage cylinder 12 (FIG. 2), thehelical gas passage 38 connecting both ends of the exteriorcircumferential surface or the interior circumferential surface.

Letting the inside of the second-stage displacer 14 through which heliumgas (a refrigerant gas) flows be a main gas passage, the helical gaspassage 38 forms an auxiliary gas passage. Further, the helical gaspassage 38 includes a groove pattern formed on the exteriorcircumferential surface of the second-stage displacer 14 or on theinterior circumferential surface of the second-stage cylinder 12 (FIG.2). This groove pattern includes a groove having at least part thereofextending along a direction to cross the axial directions (verticaldirections in FIG. 2 and FIG. 3) of the second-stage displacer 14 so asto cause the helium gas to actively exchange heat with the second-stagecylinder 12 and the second-stage displacer 14 in the gap between thesecond-stage displacer 14 and the second-stage cylinder 12, the heliumgas flowing from one end to the other end of the exteriorcircumferential surface or the interior circumferential surface. FIG. 2and FIG. 3 illustrates a case where the helical gas passage 38 is formedon the exterior circumferential surface of the of the second-stagedisplacer 14 of the two-stage GM refrigerator.

FIGS. 5A and 5B, 6A and 6B, 7A and 7B, and 8A and 8B illustrate thecooling characteristics of the two-stage GM refrigerator according tothis embodiment (Example) and a conventional two-stage GM refrigerator(Comparative Example) in comparison. The conventional two-stage GMrefrigerator used employs lead as a regenerator material and has asealing ring to control gas flow provided between the second-stagecylinder and the second-stage displacer. In FIGS. 5A through 6B, thetwo-stage GM refrigerator according to this embodiment is indicated as“Bi+helix,” and the conventional two-stage GM refrigerator is indicatedas “Pb+sealing ring.”

Further, FIGS. 5A through 6B illustrate characteristics at an operatingfrequency of 50 Hz, and FIGS. 7A through 8B illustrate characteristicsat an operating frequency of 60 Hz. Further, FIGS. 5A and 5B and 7A and7B illustrate characteristics at the time of no load on either thefirst-stage heat station or the second-stage heat station. FIGS. 6A and6B and 8A and 8B illustrate characteristics at the time of applying aload of 12 W on the first-stage heat station and a load of 3 W on thesecond-stage heat station. Further, FIGS. 5A, 6A, 7A, and 8A illustratethe temperature characteristics of the first-stage heat stations, andFIGS. 5B, 6B, 7B, and 8B illustrate the temperature characteristics ofthe second-stage heat stations.

In no-load operations, the first-stage temperatures of the Example andthe Comparative Example are substantially the same as illustrated inFIG. 5A and FIG. 7A. On the other hand, as illustrated in FIG. 5B andFIG. 7B, while the second-stage temperature of the Comparative Exampleis 6.5 K to 7.2 K, the second-stage temperature of the Example is 5.3 Kto 5.5 K. Thus, there is improvement in the temperature characteristicof Example with respect to the second stage.

In the loaded operations (at 50 Hz) illustrated in FIGS. 6A and 6B, thefirst-stage temperature of the Comparative Example is 71 K to 80 K,while the first-stage temperature of the Example is 65 K to 66 K asillustrated in FIG. 6A. Thus, there is improvement in the temperaturecharacteristic of the Example with respect to the first stage. Further,the second-stage temperature of the Comparative Example is 10.1 K to11.0 K, while the second-stage temperature of the Example is 9.5 K to9.8 K as illustrated in FIG. 6B. Thus, there is improvement in thetemperature characteristic of the Example with respect to the secondstage.

In the loaded operations (at 60 Hz) illustrated in FIGS. 8A and 8B, thefirst-stage temperature of the Comparative Example is 65 K to 78 K,while the first-stage temperature of the Example is 62 K to 63 K asillustrated in FIG. 8A. Thus, there is improvement in the temperaturecharacteristic of the Example with respect to the first stage. Further,the second-stage temperature of the Comparative Example is 9.8 K to 10.7K, while the second-stage temperature of the Example is 9.2 K to 9.4 Kas illustrated in FIG. 8B. Thus, there is improvement in the temperaturecharacteristic of the Example with respect to the second stage.

It is believed to be for the following reason that the two-stage GMrefrigerator according to this embodiment shows a good coolingcharacteristic relative to the Comparative Example.

According to the two-stage GM refrigerator according to this embodiment,a groove pattern forming the helical gas passage 38 is formed on, forexample, the exterior circumferential surface of the second-stagedisplacer 14. This causes helium gas (a refrigerant gas) to diverge fromthe main gas passage passing through the second-stage displacer 14 so asto flow through the helical gas passage 38 formed between thesecond-stage displacer 14 and the second-stage cylinder 12.

The groove pattern forming the helical gas passage 38 is so formed as toinclude a groove along a direction to cross the axial directions of thesecond-stage displacer 14 so as to cause the helium gas flowing throughthe groove to actively exchange heat with the second-stage displacer 14and the second-stage cylinder 12.

Therefore, when the helium gas, which is a refrigerant gas, flows fromthe lower-temperature side to the higher-temperature side, the heliumgas cools the second-stage displacer 14 and the second-stage cylinder 12more efficiently than conventionally. As a result, the bismuthregenerator material 18 filling in the second-stage displacer 14 iscooled with more efficiency than in the conventional configurationwithout the helical gas passage 38. On the other hand, when the divergedhelium gas flows from the higher-temperature side to thelower-temperature side, the helium gas is more cooled than in the caseof flowing directly in the axial direction. Accordingly, it is believedthat it is possible to improve the cooling efficiency by providing thehelical gas passage 38 even if bismuth, which is lower in specific heatthan lead, is used as the regenerator material 18 at cryogenictemperatures lower than or equal to 15 K.

FIG. 9 is a graph illustrating a relationship between the bismuth size(grain size) and the refrigerating capacity of the bismuth regeneratormaterial 18. FIG. 9 shows that if the grain size is less than 0.14 mm,the second-stage displacer 14 is filled with bismuth with excessivelyhigh density so as to cause a sharp increase in the resistance topassage of helium gas, which is a refrigerant gas. On the other hand, ifthe grain size exceeds 1.6 mm, there may be a significant decrease inthe efficiency of heat exchange between the bismuth regenerator material18 and the helium gas and the second-stage displace 14. Accordingly, thebismuth granules are desirably more than or equal to 0.14 mm and lessthan or equal to 1.6 mm in grain size.

FIG. 10 is a diagram illustrating another configuration of thesecond-stage displacer 14 according to this embodiment. According tothis configuration, the cylindrical member 30 includes a cylindricalstainless steel tube 39 and a wear-resistant resin member 40 fixed ontothe surface of the stainless steel tube 39. The wear-resistant resinmember 40 is formed of fabric-containing phenolic resin.

For example, the wear-resistant resin member 40 is slightly smaller than35 mm in outside diameter and 32 mm in inside diameter, and thestainless steel tube 39 is 30 mm in inside diameter. The stainless steeltube 39 having high mechanical strength is provided inside thewear-resistant resin member 40 to control the thermal contraction of thewear-resistant resin member 40 at the time of cooling. As a result, theheat distortion properties of the second-stage displacer 14 approach theheat distortion properties of the stainless steel tube 39.

A lid member 41 having a circular ring shape is inserted into thecylindrical member 30 at its upper open end, but otherwise theconfiguration of the second-stage displacer 14 is the same as theconfiguration illustrated in FIG. 3. The configurations as illustratedin FIG. 3 and FIG. 10 make it unnecessary for the second-stage displacer14 to contain a sealing ring. Accordingly, it is possible to reduce thethickness of the sidewall of the cylindrical member 30.

This means a possible increase in the space for containing the bismuthregenerator material 18 in the second-stage displacer 14. An increase inthe amount of bismuth leads to an increase in the refrigeratingcapacity. In particular, in the case of using bismuth, which is lower inspecific heat than lead, as the regenerator material 18, this increasein the bismuth regenerator material 18 is advantageous in terms ofimproving the cooling capacity.

Although a description is given above of the case of providing thehelical gas passage 38 only on the second-stage displacer 14 in theregenerative refrigerator illustrated in FIG. 2, it is also possible toprovide a helical gas passage on the first-stage displacer 13. FIG. 11illustrates a configuration of the first-stage displacer 13 having ahelical gas passage 55 provided on its cylindrical exteriorcircumferential surface.

Referring to FIG. 11, the first-stage displacer 13 includes acylindrical member 50 formed of fabric-containing phenolic resin. Thecylindrical member 50 has a cylindrical shape with an upper lid (notgraphically illustrated), and is open at its lower end. A flange 51,whose diameter is slightly smaller than the outside diameter of thecylindrical member 50, is attached to the upper surface of the upper lidof the cylindrical member 50. An opening 52 that forms a gas passage isprovided through the flange 51 and the upper lid of the cylindricalmember 50. The drive shaft S for driving the cylindrical member 50 inthe vertical directions indicated by a double-headed arrow in FIG. 11 isattached to the upper surface of the flange 51.

In the cylindrical member 50, the regenerator material 17 such as a wiremesh of copper fills in a space between an upper wire mesh and a lowerwire mesh (both of which are not graphically illustrated). That is, theupper wire mesh is placed on the regenerator material 17, and the lowerwire mesh is placed under the regenerator material 17. Openings 53 forforming a gas passage are formed in the sidewall of the cylindricalmember 50 at the same vertical position as where the lower mesh wire isplaced under the regenerator material 17.

Further, a lid member 54 formed of fabric-containing phenolic resin orthe like is inserted into and bonded to the cylindrical member 50 at itsopen lower end. The lid member 54, which is a blank cap, hermeticallyseals the lower-end opening of the cylindrical member 50. Further, arecess for attaching the joining mechanism 36 (FIG. 3) for connection tothe second-stage displacer 14 is formed on the lower surface of the lidmember 54.

The helical gas passage 55 formed of a single helical groove is formedon the exterior circumferential (circumferential) surface of thecylindrical member 50 from its upper end to a vertical position wherethe openings 53 are formed. The cylindrical member 50 is slightlysmaller in outside diameter below the vertical position of the openings53 than above the vertical position of the openings 53. Accordingly, agap is formed between the cylindrical interior circumferential surfaceof the first-stage cylinder 11 (FIG. 2) and the exterior circumferentialsurface of the cylindrical member 50 below the vertical position of theopenings 53. This gap serves as a gas passage connecting the inside ofthe cylindrical member 50 and the first-stage expansion space 21illustrated in FIG. 2.

The diameter of the flange 51 is slightly smaller than the outsidediameter of the cylindrical member 50. Therefore, a gap is formedbetween the exterior circumferential surface of the flange 51 and theinterior circumferential surface of the first-stage cylinder 11. Thisgap serves as a gas passage connecting the openings 53 (gas passage) andthe upper space inside the first-stage cylinder 11 illustrated in FIG.2. Thus, providing the first-stage displacer 13 with the helical gaspassage 55 makes it possible to improve the cooling characteristic inthe first stage for the same reasons as the cooling characteristic isimproved in the second stage as described above.

Further, in the above-described embodiments, a description is given ofthe case of forming the helical gas passage (4, 38, 55) on the surfaceof the displacer (2, 14, 13). However, the shape of the helical gaspassage is not limited to a helical shape and may be other shapes aslong as the shapes allow helium gas to flow through a gap between thecylinder (1, 12, 11) and the displacer (2, 14, 13) while exchanging heatsufficiently with the surface of the gas passage. A description is givenbelow, with reference to FIGS. 12A through 12H, of other gas passageshapes.

FIGS. 12A through 12H are schematic diagrams illustratingcircumferentially developed groove patterns formed on the exteriorcircumferential surfaces of the displacers. FIGS. 12A through 12Hillustrate the characteristics of the shapes of groove patterns, and donot illustrate a groove pitch or a groove inclination relative to theaxial directions.

FIG. 12A illustrates the case where a single helical groove is formed onthe exterior circumferential surface of a displacer from its one end tothe other end. Alternatively, multiple helical grooves may be providedas illustrated in FIG. 12B. FIG. 12B illustrates the case where fourgrooves are formed substantially parallel to one another. Further, thehelical groove may be formed in a wavy line as illustrated in FIG. 12Cor a zigzag line as illustrated in FIG. 12D. Further, as illustrated inFIG. 12E, the helical groove may be formed in a step-like zigzag line bycombining straight lines parallel to and straight lines perpendicular tothe axial directions of the displacer. Further, as illustrated in FIG.12F, a wavy line and a zigzag line may be combined.

As illustrated in FIG. 12G, two or more helixes that spiral indirections opposite to each other may be combined so that helicalgrooves cross each other. Further, as illustrated in FIG. 12H, multiplegrooves may be formed circumferentially on the exterior circumferentialsurface of a displacer, and connection grooves that connect adjacentcircumferential grooves may be formed. In this case, in order to form aslong a gas passage as possible, it is preferable to provide theconnection grooves, formed vertically between the circumferentialgrooves, at different circumferential positions. Further, it ispreferable to position the connection grooves axisymmetrically.

Thus, a groove pattern is formed so that at least part of one or moregrooves thereof extends along a direction to cross the axial directionsof a displacer. As a result, gas flows through a longer passage than inthe case of flowing parallel to the axial directions. This allows heatexchange to be performed with more efficiency between the gas and thedisplacer and the cylinder.

The cross section of the gas passage formed on the exteriorcircumferential surface of a displacer may be rectangular, triangular,semicircular, or of other shapes. Further, in order to increase the heatexchange efficiency of the gas flowing through the gas passage formed onthe exterior circumferential surface of a displacer, a regeneratormaterial may be stuck to the exterior circumferential surface of thedisplacer or the internal surface of the gas passage. Further, the gaspassage may be filled with a regenerator material.

In the above-described embodiments, a description is given of the caseof forming a groove pattern on the exterior circumferential surface of adisplacer (2, 13, 14). However, the same effects may be produced byforming a groove pattern on the interior circumferential surface of acylinder (1, 11, 12). In this case, the groove pattern may be formed toconnect both ends of a cylindrical region of the interiorcircumferential surface of the cylinder, the cylindrical regionincluding at least a range over which the displacer reciprocates.

FIG. 13 is a diagram illustrating a basic configuration of the cylinder1 and the displacer 2 in the case where a groove pattern is formed onthe cylindrical interior circumferential surface of the cylinder 1. Ahelical gas passage 4 a is formed on the interior circumferentialsurface of the cylinder 1 in place of the helical gas passage 4 formedon the exterior circumferential surface of the displacer 2 (FIG. 1).Otherwise, the configuration of FIG. 13 is the same as the basicconfiguration of FIG. 1. Various groove patterns such as thoseillustrated in FIGS. 12A through 12H may be formed in place of thehelical groove pattern.

According to one aspect of the present invention, a regenerativerefrigerator uses bismuth as a regenerator material. Accordingly, it ispossible to reduce burdens on the environment.

Further, a groove pattern may be formed on the cylindrical exteriorcircumferential surface of a displacer, so that gas that diverges from amain gas passage containing a regenerator material flows along thisgroove pattern in a gap between the displacer and a cylinder. Thisgroove pattern is formed to include a groove extending along a directionto cross the axial directions of the displacer so as to allow the gasflowing in the groove to actively exchange heat with the displacer andthe cylinder.

Accordingly, when the diverged gas flows from the higher-temperatureside to the lower-temperature side, the gas is more cooled than in thecase of flowing directly in the axial direction. On the other hand, whenthe diverged gas flows from the lower-temperature side to thehigher-temperature side, the gas cools the displacer and the cylindermore. Accordingly, it is possible to ensure attainment of cryogenictemperatures lower than or equal to 15 K even with bismuth, which islower in specific heat than lead conventionally used as a regeneratormaterial.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventors to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority orinferiority of the invention.

Although the embodiments of the present inventions have been describedin detail, the present invention is not limited to those specificallydisclosed embodiments. For example, the present invention is applicableto not only GM refrigerators but also other refrigerators usingregenerators, such as Stirling refrigerators and Solvay cyclerefrigerators.

Further, the above description is given, taking a two-stage displacerconfiguration as an example. However, the present invention may also beapplied to the case of using a single stage displacer or three or morestage displacers. Further, in other configurations, the presentinvention may be applied to regenerative refrigerators using displacersat low temperatures. It should be understood that various changes,substitutions, and alterations could be made hereto without departingfrom the spirit and scope of the invention.

1. A regenerative refrigerator, comprising: a cylinder formed of amaterial having a low thermal conductivity and a high airtightness, thecylinder having a cylindrical interior circumferential surface; adisplace provided in the cylinder so as to be reciprocatable in axialdirections thereof with an expansion space formed between one end of thecylinder and the displacer, the displacer having an exteriorcircumferential surface along a cylindrical shape of the interiorcircumferential surface of the cylinder, the exterior circumferentialsurface being slightly smaller in diameter than the interiorcircumferential surface; a groove pattern formed on one of the exteriorcircumferential surface of the displacer and the interiorcircumferential surface of the cylinder so as to form a first gaspassage connecting a first end and a second end of the one of theexterior circumferential surface of the displacer and the interiorcircumferential surface of the cylinder, the groove pattern including agroove having at least a part thereof extending along a direction tocross the axial directions of the displacer so as to cause a gas flowingfrom one to another of the first end and the second end of the one ofthe exterior circumferential surface of the displacer and the interiorcircumferential surface of the cylinder in a gap between the exteriorcircumferential surface of the displacer and the interiorcircumferential surface of the cylinder to actively exchange heat withthe cylinder and the displacer; a second gas passage through which thegas is supplied to and collected from the expansion space; and aregenerator material formed of bismuth granules and provided in at leasta part of the second gas passage, wherein a lowest attainabletemperature of the regenerative refrigerator is in a range of cryogenictemperatures higher than or equal to 5 K and lower than or equal to 15 Kin an unloaded state.
 2. The regenerative refrigerator as claimed inclaim 1, wherein the groove pattern has a helical shape.
 3. Theregenerative refrigerator as claimed in claim 2, wherein the helicalshape of the groove pattern is formed with multiple helixes arranged inparallel.
 4. The regenerative refrigerator as claimed in claim 1,wherein the displacer is hollow with an internal cavity, the internalcavity being filled with the regenerator material to form the second gaspassage.
 5. The regenerative refrigerator as claimed in claim 1, whereinthe gap between the exterior circumferential surface of the displacerand the interior circumferential surface of the cylinder is 0.01 mm to0.03 mm.
 6. The regenerative refrigerator as claimed in claim 1, whereinthe bismuth granules are greater than or equal to 0.14 mm and smallerthan or equal to 1.6 mm in grain size.